The invention relates to a dust filter with a vertically oriented filter housing and a partition dividing the filter housing into a lower dust-contaminated air chamber and an upper clean air chamber. The partition is provided with a plurality of openings in which self-supporting filter tubes are suspended which are open in the upward direction to the clean air chamber, wherein means are provided to clean off the dust and product particles adhering to the outer surfaces of the filter tubes.
Such a dust filter is known from DE 44 40 279 A1. For dust removal filter elements in the form of filter hoses or filter tubes are often used which have a diameter of approximately 150 mm and a length of 4 m and more. In addition to the filter hoses suspended from the partition by their own weight, it is known to employ self-supporting filter tubes for forming the filter elements. In both cases the material for the filter elements is needled felt. The filter elements are arranged in a dust-contaminated air chamber, and the dust-contaminated air flows through them from the exterior to the interior. Their inner space is in communication with the clean air chamber which is operated at a reduced pressure relative to the dust-contaminated air chamber, and the cleaned gas exits the filter device from the clean air chamber. The dust and product particles which collect on the outer walls of the filter elements form a dust cake whose thickness increases during the course of operation. For this reason, in the known dust-contaminated air filters cleaning means are provided in order to clean off the adhering dust and product particles from the outer surfaces of the filter elements.
For example, it is known to clean the filter elements by means of a compressed air pulse injector systems so that the dust or product particles adhering to the outer side of the hose are blasted off by backwashing with compressed air.
When the dust filters are used in production devices which are employed for different products, it is required to clean off the dust or product residues, which inevitably adhere during the filter process to the filter hoses and inner walls of the filter housing, from the entire dust-contaminated air chamber of the filters, including the filter hoses or filter tubes, such that product corruption and product contamination are prevented in the case of a changeover of a product. Such a hygiene-complying operation is mandatory primarily in the processing of foodstuffs, pharmaceutical and medical products.
For cleaning dust filters it is also known to clean the outer surfaces of the filter hoses or filter tubes as well as the inner side of the filter housing with a washing liquid which is dispensed by washing nozzles arranged in the dust-contaminated air chamber.
The dust filters known in the prior art have the disadvantage that, in addition to requiring a large space for constructive lengths of the filter hoses or filter tubes of more than 4 m length, they can be cleaned in a hygiene-complying way only with great expenditure. The filter materials employed in the known dust filters have a thickness of approximately 1.5 to 2 mm for ensuring a sufficient mechanical strength. This relatively thick porous material will become saturated with washing liquid during the washing process and must be dried in a time-consuming drying process before reusing the dust filter.
Based on this, the object of the invention is to further develop a dust filter of the aforementioned kind such that the dust filter, while requiring only minimal space, requirement enables a simple and quick cleaning of the filter elements.
The solution to this object is characterized according to the invention in that several filter tubes are combined to a filter bundle wherein the spacing between the individual filter tubes of each filter bundle is smaller than the spacing between individual filter bundles relative to one another.
Combining several filter tubes with a diameter of less than 100 mm, preferably 40 to 50 mm, to a filter bundle allows to reduce the constructive length of the filter tubes to less than 4 m, preferably approximately 2 m. The material for the filter tubes is preferably a self-supporting needled felt with a very smooth surface, high air permeability, and a wall thickness of less than 1 mm, preferably approximately 0.6 mm. In this way it is possible to achieve that, by means of the filter bundles that are relatively short in comparison to the prior art, a throughput of air to be filtered can be achieved which compares to that of the very long filter tubes known in the prior art.
According to a preferred embodiment of the invention, a compressed air source of a compressed air pulse injector system is arranged as a cleaning means in the clean air chamber above each filter bundle in order to remove by compressed air pulses the dust and product particles adhering to the outer surface of the filter tubes. In this connection, by combining several filter tubes to a filter bundle it is sufficient to provide only one compressed air source for each filter bundle.
In order to concentrate the compressed air pulses ejected by the compressed air pulse injector system onto the individual filter tubes of a filter bundle, it is moreover suggested that each filter bundle at the top side of the partition is surrounded by a catch nozzle enveloping the filter tubes of the respective filter bundle. With this catch nozzle, whose inner diameter is advantageously somewhat greater than a diameter of a trace circle surrounding the filter tubes of the filter bundle, the compressed air pulses applied by the compressed air source are directed in a targeted way into the individual filter tubes of each filter bundle so that the backwashing action of the compressed air pulses is enhanced.
The air throughput upon backwashing of the filter tubes by means of the compressed air pulse injector system can be increased moreover in that the compressed air cleaning of the filter tubes is realized in addition by means of secondary air that is taken in at least in one area of the injector system. This secondary air is taken in and entrained by means of the compressed air pulses ejected by the compressed air source so that the total air throughput upon backwashing of the filter tubes is comprised of the compressed air of the compressed air source as well as the additionally taken in secondary air.
Particularly when employing dust filters according to the invention in processing of foodstuffs and/or medical and pharmaceutical products, it is unavoidable to perform also a complete cleaning and removal of the separated dust and product particles in the dust-contaminated air chamber. For this purpose, washing nozzles for a directed dispensing of washing liquid are provided in the dust-contaminated air chamber as cleaning means for cleaning the filter tubes wherein the washing nozzles are arranged at the points of intersection of diagonals extending from the center points of neighboring filter bundles.
With this arrangement of the washing nozzles, which end preferably flush with the underside of the partition and from which the washing liquid exits essentially radially, it is ensured according to a preferred embodiment of the invention that the washing liquid flows through the individual passages between the filter bundles as well as between the filter tubes of a filter bundle as well as impinges on the outer surface of the filter tubes. With this arrangement according to the invention of the washing nozzles, a complete cleaning of the outer surfaces of the filter tubes of each filter bundle is ensured.
For a complete cleaning of the dust-contaminated chamber it is furthermore suggested that the washing nozzles are arranged such that the washing liquid exiting from the washing nozzles cleans the inner side of the filter housing.
According to a practical embodiment of the invention, the filter housing is surrounded in the lower area of the filter tubes at the outer side by an annular channel into which a dust-contaminated air supply channel opens tangentially for supplying the dust-contaminated air into the filter housing. With this indirect introduction of the dust-contaminated air into the filter housing by means of the annular channel surrounding the filter housing, the mechanical damage of the material of the filter tubes, known in the prior art in the case of direct introduction of dust-contaminated air into the cylinder housing, is avoided. Moreover, this indirect introduction of the dust-contaminated air has the advantage that the speed of the dust-contaminated air to be introduced must not be reduced by suitable constructive measures because the risk of damages of the filter elements is not present.
For obtaining a complete cleaning of the entire filter housing, it is furthermore suggested that also in the area of the entry of the dust-contaminated air into the annular channel from the dust-contaminated air supply channel and/or in the annular channel at least one washing nozzle is provided for a directed dispensing of washing liquid. Only by means of this additional arrangement of washing nozzles in the area of the dust-contaminated air supply it can be ensured that the dust or product residues of the previous product are completely removed from the filter housing beforehand in the case of a product changeover.
According to a practical embodiment of the invention, the dispensing of the washing liquid via the washing nozzles is realized by means of a washing system comprising several liquid circuits. By distributing the washing liquid into different liquid circuits, the washing liquid can be supplied in a directed way into individual washing nozzles in different sections of the filter housing. The control of the liquid circuits and the washing system is realized advantageously by a washing program control.
Finally, it is proposed with the invention to form a filter bundle of seven filter tubes.